Hydrology, Hydraulics & Flood Insurance Studies

Hydrology, Hydraulics & Flood Insurance Studies

Introduction to Riverine Science

Floodplain management relies on sound engineering and scientific principles to quantify flood risks. To manage a floodplain effectively, professionals must understand how water moves across the land and through river channels. This foundation is built upon two distinct but closely related engineering disciplines: Hydrology and Hydraulics. While they sound similar, they answer two very different questions about a flood event.

Hydrology: The Study of Runoff

Hydrology is the study of water's occurrence, distribution, and movement. In the context of floodplain mapping, hydrology specifically focuses on surface water runoff. When a rainstorm occurs, water falls on a watershed (the area of land that drains to a single point). Some of this water infiltrates the soil, some evaporates, and the rest becomes surface runoff.

Hydrologists analyze watersheds to answer a primary question: How much water will flow through this river during a specific storm event? They calculate the peak discharge, which is the maximum volume of water passing a specific point per unit of time. In the United States, peak discharge is measured in cubic feet per second (cfs).

Several watershed characteristics influence hydrology:

• Drainage Area: Larger watersheds naturally collect more rainfall, leading to higher peak discharges.
• Topography: Steep slopes cause water to flow faster, concentrating runoff quickly and creating sharp, high flood peaks.
• Soil and Land Use: Highly permeable soils (like sand) absorb more water, reducing runoff. Conversely, urban development introduces impervious surfaces (concrete, asphalt, roofs) that prevent infiltration, drastically increasing both the volume and speed of runoff.
• Precipitation Data: Hydrologists rely on historical rainfall data to model different storm frequencies and intensities.

Hydraulics: The Study of Water Level and Velocity

While hydrology determines the amount of water (cfs), hydraulics determines how that water behaves within the channel and floodplain. Hydraulic engineers take the peak discharge calculated by hydrologists and ask: Given this amount of water, how high will the water rise, and how fast will it move?

Hydraulic analysis considers the physical constraints of the river system. Key factors include:

• Channel Cross-Section: The shape and size of the river channel and adjacent floodplain dictate how much water can be conveyed at a given depth. A wide, flat floodplain will spread the water out, resulting in a lower flood elevation. A narrow gorge will force the water to rise significantly.
• Roughness (Manning's 'n' value): As water flows, it encounters friction from the stream bed, banks, and floodplain surface. Smooth surfaces (like a concrete channel) have low roughness, allowing water to flow quickly. Dense vegetation, trees, and buildings have high roughness, slowing the water down and causing it to back up and rise higher.
• Slope: The longitudinal slope of the riverbed affects velocity. Steeper rivers have faster-moving water.
• Obstructions: Bridges, culverts, dams, and levees alter the flow of water. A culvert that is too small for a major flood will act as a dam, causing significant upstream flooding.

By combining hydrologic discharge data with hydraulic channel characteristics, engineers calculate the Base Flood Elevation (BFE), which is the regulatory water surface elevation for the National Flood Insurance Program (NFIP).

The 1%-Annual-Chance Flood (100-Year Flood)

The cornerstone of the NFIP is the 1%-annual-chance flood. This is the flood event that has a 1 in 100 (or 1%) chance of being equaled or exceeded in any given year. The area inundated by this flood is designated on FEMA maps as the Special Flood Hazard Area (SFHA).

Historically, this has been referred to as the "100-year flood." However, floodplain managers strongly discourage this term because it creates a false sense of security. Many people mistakenly believe that if they experience a "100-year flood," they are safe for the next 99 years. In reality, the 1% probability resets every single year. It is entirely possible to experience multiple 1%-annual-chance floods in a single decade.

To communicate this risk better, floodplain managers often frame it over the life of a standard 30-year mortgage. A structure located within the SFHA has a 26% chance of experiencing a 1%-annual-chance flood during a 30-year period. This highlights the cumulative risk over time.

Flood Insurance Studies (FIS)

The detailed engineering analyses described above are compiled into a document called a Flood Insurance Study (FIS). The FIS report provides the technical data that supports the visual Flood Insurance Rate Map (FIRM). While the map shows where the flood zones are, the FIS explains how those zones were determined.

An FIS report typically contains several crucial components:

1. The Text Report

The narrative portion of the FIS provides the background of the study. It describes the community, the specific flooding sources (rivers, lakes, coastlines) that were studied, and historical flood events. Most importantly, it details the exact engineering methods and models used for the hydrologic and hydraulic analyses.

2. Summary of Discharges Table

This table is the primary output of the hydrologic analysis. It lists the peak discharge rates (in cfs) for various flood frequencies (typically the 10%, 2%, 1%, and 0.2% annual-chance floods) at specific locations along the studied streams.

3. Flood Profiles

Flood profiles are graphical representations of the water surface elevations along a stream. They display the elevation of the stream bed, the locations of bridges and culverts, and the water surface elevations for different flood frequencies (usually the 10-, 50-, 100-, and 500-year floods).

Flood profiles are critical for floodplain managers because they provide the exact, unrounded Base Flood Elevation at any point along a studied stream. FIRMs only show rounded BFE values (to the nearest whole foot or tenth of a foot), so the flood profile is the definitive source for regulatory elevations.

4. Floodway Data Table

For streams where a regulatory floodway has been established, the FIS includes a Floodway Data Table. This table provides detailed hydraulic data at each cross-section, including the floodway width, section area, mean velocity, and the Base Flood Elevation both with and without the floodway encroachment.

Hydraulic Modeling: HEC-RAS

The industry standard software for modeling riverine hydraulics in the United States is the Hydrologic Engineering Center's River Analysis System (HEC-RAS), developed by the U.S. Army Corps of Engineers.

Engineers use HEC-RAS to simulate how water flows through natural channels. They input the channel geometry by defining cross-sections along the river. They assign roughness coefficients based on land cover, and they input the peak discharge values generated from the hydrologic analysis. HEC-RAS then performs complex hydraulic calculations to determine the water surface profile, effectively mapping the Base Flood Elevations from one cross-section to the next.

While floodplain managers do not need to be expert HEC-RAS modelers, they must understand its basic inputs and outputs. Recognizing how changes in the floodplain—such as adding fill or constructing a bridge—alter the HEC-RAS model is essential for evaluating development permits and ensuring compliance with NFIP regulations.